J. Phys. Chem. C 2007, 111, 3467-3470
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Synthesis of Layer-Deposited Silicon Nanowires, Modification with Pd Nanoparticles, and Their Excellent Catalytic Activity and Stability in the Reduction of Methylene Blue Hui Hu, Mingwang Shao,* Wu Zhang,* Lei Lu, Hong Wang, and Sheng Wang Anhui Key Laboratory of Functional Molecular Solids, and College of Chemistry and Materials Science, Anhui Normal UniVersity, Wuhu 241000, People’s Republic of China ReceiVed: October 1, 2006; In Final Form: January 9, 2007
Agglomeration of nanoparticles during the preparation and utilization of nanocatalysts remains a problem. Here, we synthesized layer-deposited silicon nanowires and modified them with Pd nanoparticles on the surface. These Pd/Si catalysts were used in the reduction of methylene blue in the presence of sodium borohydride, which enhanced reaction rate by 30 times compared with unsupported Pd nanoparticles. They were also provided with more excellent stability than polymer-supported Pd. The catalytic rate of the catalysts only decreased by 20% when the cyclic time was up to 50.
1. Introduction Stabilized cluster and colloids of noble metals such as Pd with nanometer-scale dimensions have been of particular interest as catalysts for organic and inorganic reaction.1-4 As their catalytic efficiency often increases as the cluster size decreases, the catalyst particles are usually obtained via stabilization in a suitable solid matrix or by surface modification using ligands5 and functionalized polymers.6-10 Yet, the agglomeration of nanoparticles during the preparation and utilization of nanocatalysts remains a formidable problem.11 During the past years, an alternative method for generating stabilized metal nanoparticles involves synthesizing them in nanoporous supports, which help define particle size and serve to immobilize the resulted particles. For example, Pd nanoparticles have been synthesized on carbon12 and metal oxide13 and silica supported14,15 and in zeolite cage.16 Herein, we present a new strategy for synthesizing Pd nanoparticles with easy recycle by using layer-deposited silicon nanowires (SiNWs) as a carrier. As a kind of typical onedimensional nanomaterial, SiNWs were produced via numerous methods,17-20 which provided a number of remarkable advantages, such as easy surface modification with various metal particles,21 a vast surface-to-volume ratio,22 and stability to atmosphere environment. In this work, layer-deposited SiNWs were successfully prepared at first and then modified with Pd nanoparticles. The Pd/Si nanostructure was employed as catalysts in the reduction of methylene blue (MB) in the presence of sodium borohydride (SB), which showed high activity and excellent stability after recycle. 2. Experimental Section Synthesis of Layer-Deposited SiNWs. The typical synthesis process was presented as follows: first, a single quartz crystal of ca. 8 cm3 was placed into an alumina boat located in the center region of tube, which was mounted inside a hightemperature tube furnace. Before heating, the furnace was vacuumized to 1 Pa and then hydrogen gas was flowed through * To whom correspondence should be addressed. Fax: +86-5533869303. Phone: +86-553-3869303. Email Address:
[email protected] (M.S.);
[email protected] (W.Z.).
the tube at a rate of 50 ccm. After that, the furnace was heated to 1350 °C with the temperature invariant for 8 h. Finally, the furnace was cooled to room temperature naturally. Among the whole process, hydrogen was kept flowing continually until the products were taken out. Modification with Pd Nanoparticles. 0.005 g as-synthesized SiNWs were etched with 5 mL 5% HF aqueous solution for 30 min, then rinsed with distilled water and immersed in 10 mL 1 × 10-3 M PdCl2 aqueous solution. When the yellow SiNWs gradually turned black ones, they were modified with Pd nanoparticles. Catalytic Reduction Process. In a representative reduction experiment, 0.1 mL of MB solution (2.5 × 10-3 M) was mixed with 2.8 mL of Na2CO3/NaHCO3 buffer solution (pH ) 9.5) and purged with N2 gas for 5-6 min to remove all dissolved oxygen. Then the as-prepared catalysts and 0.15 mL (0.1, 0.01, 1 × 10-3, 1 × 10-4, 1 × 10-5 M) of freshly prepared SB were added. The progress of the reaction was monitored using a spectrophotometer. Characterization. The phase purity of the as-prepared products was determined by a Shimadzu XRD-6000 X-ray diffractometer equipped with Cu KR radiation (λ ) 0.15406 nm). A scanning rate of 0.05 ° s-1 was applied to record the pattern. Scanning electron microscopy (SEM) images were obtained on an X-600 scanning electron microscope. The nanostructure of the catalyst was further observed by transmission electron microscopy (TEM) and high-resolution transmission electron microscopy (HRTEM), which were taken on a JEOL-2010 transmission electron microscope, with an accelerating voltage of 200 kV. Reduction process was tracked with a U-4100 spectrophotometer. 3. Results and Discussion Synthesis and Modification of SiNWs. Figure 1a shows the X-ray diffraction (XRD) pattern of the as-prepared SiNWs, and all diffraction peaks can be indexed as the cubic phase of Si. The cell parameter is calculated to be a ) 0.5421 ( 0.0005 nm, which is in agreement with the value of face-centered cubic silicon a ) 0.5430 nm (Joint Committee of Powder Diffraction Standards, JCPDS card no. 27-1402). The SiNW-supported Pd catalysts are characterized with XRD (Figure 1b). No other
10.1021/jp066440f CCC: $37.00 © 2007 American Chemical Society Published on Web 02/03/2007
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Figure 1. XRD patterns of (a) SiNWs and (b) Pd/Si nanostructure.
Figure 3. TEM image of a single SiNW supported with Pd nanoparticles and the comparative one (inset left nether) of the recycled SiNWs indicating the diameter of the Pd nanoparticles without any obvious change after repetitious catalytic reduction, SAED pattern (inset right nether) indicating bright diffraction spots of Si and weak diffraction ring of Pd, HRTEM image (inset left above) showing clear (111) crystal planes of Si and Pd, and EDX spectrum (inset right above) of the Si and Pd peaks revealing atomic ratio of 4.5:2.4.
Figure 2. SEM image of SiNWs revealing layer-deposited morphology, the EDX spectrum showing the atomic ratio of Si to O of 91.1:8.9.
characteristic peaks were observed except Pd and Si. The XRD pattern indicates that the product has a high degree of crystallinity. The cell parameter of Pd is calculated to be a ) 0.3887 ( 0.0003 nm, which is in accordance with the JCPDS data (a ) 0.3890 nm from JCPDS card no. 46-1043). The SEM image (Figure 2) reveals that the sample takes the shape of a wire, which is smooth and uniform with the average diameter of 65 nm, and it also unambiguously suggests that SiNWs are deposited layer by layer in order, which is useful in the fabrication of Pd/Si catalysts. The energy dispersive X-ray spectra (EDX) of SiNWs (Figure 2, inset) show no peaks of other elements except Si and O, indicating the high purity of the product, whereas other SiNWs usually have thick silicon oxide sheath, the thickness of which is one-quarter to one-third of the wire diameter.23 The peak of Si is much higher while that of O is quite weaker. The atomic ratio of silicon to oxygen is close to 91.1:8.9 calculated from the spectrum. Figure 3 presents a TEM image of one single wire modified with Pd nanoparticles, which are randomly attached to the surface of the SiNW, taking an average diameter of 8 nm. Close examination of the sample using HRTEM (Figure 3, inset) reveals the clear lattice fringes of Pd and Si, both showing (111) crystal planes. The selected area electron diffraction (SAED) (Figure 3, inset) taken from this area displays the crystalline structure of Si and Pd. The bright spots can be indexed as (220) and (13h1) of silicon respectively, while the diffraction ring is corresponding with the (111) plane of Pd. The EDX spectrum (Figure 3, inset) shows the atomic ratio of Si:Pd of the nanowire is 4.5:2.4.
All of the above characterization results are consistent with each other and adequately prove the fact that the SiNWs have been modified with Pd nanoparticles. Catalytic Reduction of Pd/Si Nanostructure. To investigate their catalytic activity, the catalysts were employed to the reduction of MB in the presence of SB. The reaction takes place very fast using Pd/Si as catalysts with SB varying from 0.1 to 0.001 M. In pace with the decrease of the SB concentration, the reaction rate slows down. Successive UV-visible spectra of the reduction of MB at SB concentrations 1 × 10-3 M are shown in Figure 4a (inset). The absorption bands for MB decrease gradually without showing any changes in shape or position of the peaks, which indicates that the MB is reduced without any other side reactions. From the composition time curves, it is necessary to first deduce the reaction order with respect to MB and, when the results of this analysis are satisfactory, to deduce the reaction order to SB and the rate constant. Figure 4a reveals that there exist linear relations between the logarithm of CMB and time under various values of CSB, which is apparent that the reduction is a first-order reaction to the concentration of MB. When the reaction order to concentration of MB is determined, the reaction order to concentration of SB may be calculated. Figure 4b shows the relationship of reaction rate vs CSB, which is proportional to the square root of CSB obtained from the slope (0.502 ( 0.06) of Figure 4b. In combination with parts a and b of Figure 4, the reduction rate of MB may be expressed as r ) 1.4360C1/2SBCMB mol/ min. In order to have a comparison, a series of contrast experiments were performed. Keeping the other condition constant and using SB to reduce MB without catalyst, the reaction does not take
Reduction of Methylene Blue
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Figure 4. (a) Curves of concentration vs time at various MB concentration and successive UV-visible spectra (inset) of the reduction of MB at SB concentrations of 1 × 10-3 M. (b) Curve of reaction rate vs concentration of SB.
Figure 5. Curve of reaction constant vs cycle times, the concentration of SB is 5 × 10-3 M.
place even though the concentration of SB is as high as 0.01 M. The result is consistent with our expectation, which adequately proves the activity of the as-prepared catalysts. By the above work, that the as-prepared Pd/Si catalysts are provided with catalytic activity is indubitable. Since the Pd nanoparticles were supported on the layerdeposited SiNWs, the catalysts can be easily recycled and initiate the next reduction after rinsing with distilled water. The reaction was studied using recycled catalysts. Along with the increase of the recycle times, the catalysts possess high activity all the time though their activity diminishes slowly. The relationship of reaction constant vs cycle times is shown in Figure 5, which may be expressed as k ) 1.4360 - 0.0053N, where k is the reaction rate constant and N is the cyclic time. The catalytic rate of the catalysts only decreases by 20% when the cyclic time is up to 50. Another group of contrast experiments were carried out using unsupported Pd or polymer-supported Pd nanoparticles (Pd/Poly) catalysts to confirm the excellent behavior of Pd/Si catalysts.
Unsupported Pd catalysts were prepared by reducing PdCl2 solution directly to Pd nanoparticles as homogeneous catalysts.24 Figure 6a shows the UV-visible spectra of the reduction in different catalysts, both under the SB concentration of 0.005 M. The absorption intensity of the system utilizing unsupported Pd nanoparticles (Figure 6a, inset) goes slightly, while the curve followed from the reduction of Pd/Si catalysts (Figure 6a) decreases fleetly. The reaction rate of the latter is as high as 30 times that of the former obtained from Figure 6b. TEM images shown in Figures parts a and b of 7may validly expatiate the obvious distinction. The average diameter generally increased by 15 nm after reduction. Unsupported Pd particles are apt to agglomerate without any carrier during the operation, which may lead to the decrease of their activity. Pd/Poly (Pd 2 wt % on Deloxan resin) were purchased from Sigma-Aldrich Co. and employed as catalysts in the same reduction system at the SB concentration of 5 × 10-3 M (5 mg). The reaction rate was equally matched with Pd/Si catalysts (shown in Figure 6b) in the first utilization. Yet, when the Pd/ Poly catalysts were recycled, it was found that the recycled catalysts lost their activity after 5 times. Parts c and d of Figure 7 exhibit the TEM images of the Pd/Poly catalysts before and after use, respectively. The polymer reunited to conglomeration after being recycled, which may be the reason for the loss of their activity. The above results obviously indicate that the catalytic activity and stability of Pd/Si catalysts are promising, which may find wider application in the catalytic field. As to the catalytic mechanism of the reduction, the Pd particle, with an intermediate redox potential value of the donor-acceptor partner, plays an important role during the electron-transfer step and acts as an electron relay system. The reduction potential of the PdII/Pdnanoparticle is lower than that of
Figure 6. (a) UV-visible spectra of the reduction by Pd/Si catalysts, unsupported Pd nanoparticles (inset) as catalysts, both under the SB concentration of 5 × 10-3 M. (b) Curves of MB concentration vs time using Pd/Si, unsupported Pd, Pd/Poly catalysts.
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Hu et al. applications in other fields. Yet, as simply comparing a newly material against a extruded commercial catalyst by measuring some reaction performance characteristics does not provide grounds for making general claims about discovering a better catalyst,25 further work is needed in the full investigation of Pd/Si catalysts. Acknowledgment. The project was supported by the National Natural Science Foundation of China (20571001) and Excellent Scholar Foundation of Anhui Province Education. References and Notes
Figure 7. TEM images of the unsupported Pd nanoparticles: (a) before the reaction and (b) recycled. TEM images of polymer supported Pd (c) before the reaction and (d) recycled.
the PdII/Pdmetal system (+0.987 V vs NHE), which depends on the particle’s size and the nature of the adsorbed ions on the particle’s surface.24 Pd nanoparticles in our experiment with an average diameter of 8 nm have a large surface-to-volume ratio compared to bulk metal. It is possible to adsorb a large number of foreign ions onto the nanoparticle surface, which may bring high activity to the as-prepared catalysts. Unsupported Pd nanoparticles are also given with this size effect, but their size may gradually grow into large during the process of reaction and thus reduce their activity, whereas the SiNWs supported Pd nanoparticles are kept from congregating and growing large because they are fixed by the SiNWs, which also makes it possible for Pd catalysts to have high efficiency during the reaction and in recycling use. 4. Conclusion In summary, layer-deposited SiNWs were prepared via the high-temperature method. The as-prepared products, after treated by HF and modified with Pd nanoparticles, provided with high catalytic activity in the process of reducing MB. Meanwhile, their good stability had also been proved by being recycled 50 times and retained 80% of original catalytic activity. The orderly morphology and easy fabrication make it possible to find
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